Crystal Engineering for Low Defect Density and High Efficiency Hybrid Chemical Vapor Deposition Grown Perovskite Solar Cells

Microscopic Mechanisms, Morphology, and Defect Formation in the Thermally Activated Crystallization of Methylammonium Lead Iodide

In this study, we unravel the atomistic mechanisms that govern the crystallization process of methylammonium lead iodide through the application of microsecond time scale molecular dynamics simulations.The findings indicate that methylammonium iodide (MAI) and lead iodide (PbI2) precursors exhibit a propensity to aggregate into a disordered film, which ultimately undergoes a thermally activated disorder-to-order transformation to achieve crystallization. Notably, the crystal evolution during the annealing process reveals morphological characteristics consistent with the Straski-Krastanov growth mode. The temperature dependence of the crystal growth rate demonstrates an activation energy of 0.37 eV, which may be ascribed to the energy required to dissociate defective Pb-I bonds and facilitate Pb diffusion. Finally, the mechanisms underlying the spontaneous generation of lead vacancies are examined, suggesting a kinetic origin for such optically active defects. In principle, the latter suggests the potential for reducing their concentration through optimization of the growth parameters.

Vapor-Solid Reaction Techniques for the Growth of Organic-Inorganic Hybrid Perovskite Thin Films

Perovskite solar cells are considered next-generation photovoltaic technology due to their remarkable advancements in power conversion efficiency. To transition this technology from the lab to industry, the method for preparing perovskite thin films must support mass production. Currently, the solution-based slot-die technique is the primary method for depositing large-area perovskite thin films. However, solution-based methods are not standard in the semiconductor industry, where vapor-based techniques are favored for their high controllability and reproducibility. The cost of vacuum facilities and the complexity of these processes hinder many researchers, resulting in vapor-based technique development lagging behind solution-based methods in device efficiency and scale. This review focuses on the progress in growing perovskite thin films using vapor-solid reaction techniques, which are believed to offer the most direct path to commercialization. By examining the crystallization and growth mechanisms of perovskite films and discussing specific optimization strategies for vapor-solid reactions, insights into future developments and challenges in fabricating perovskite solar cells using fully vacuum processes are concluded.

Ionic Liquid-Assisted Strategy for Morphology Engineering of Inorganic Cesium-Based Perovskite Thin Films Toward High-Performance Solar Cells

The wide bandgap CsPbI2Br perovskite materials have attracted significant attention due to their high thermal stability and compatibility with narrow bandgap materials in tandem devices. The performance of perovskite solar cells (PSCs) is highly dependent on the quality of the perovskite layer, which is governed by the crystallization process during solution processing. However, the crystallization dynamics of CsPbI2Br thin films remain less explored compared to conventional organic-inorganic perovskites. Achieving high-quality CsPbI2Br films with uniform morphology and large perovskite grains remains challenging with standard solution techniques. This study applies the ionic liquid (IL) [EMIM]+[PF6]- as an additive within the bulk CsPbI2Br absorber layer. Within our experimental regime, [EMIM]+[PF6]- accelerates the crystallization process while promoting the formation of large perovskite grains, a feature not commonly observed in previous studies. Our experimental results suggest that the IL acts as heterogeneous nucleation sites, and varying IL incorporation amount significantly impacts the morphology of CsPbI2Br perovskite films. Consistent UV-vis and photoluminescence (PL) red-shifts are observed in the IL-incorporated CsPbI2Br films, with X-ray diffraction (XRD) data projecting an influence on the perovskite crystal structure. These findings provide new insights into the role of ILs in controlling crystallization and morphology that have been minimally discussed in the literature. The incorporation of an optimized amount of [EMIM]+[PF6]- promotes the formation of highly crystalline perovskite thin films with excellent morphology, reducing defect density, enhancing carrier transport, and yielding large grain sizes. As a result, PSCs fabricated with [EMIM]+[PF6]- achieved a power conversion efficiency (PCE) of 17.11% (stabilized at 15.87%) and an open-circuit voltage (VOC) of 1.39 V, along with improved stability compared to control devices. This work provides a straightforward approach for producing high-quality CsPbI2Br thin films with high reproducibility, contributing valuable advancements to Cs-based PSCs.

Imidazoanthraquinone Derivative as a Surface Passivator for Enhanced and Stable Perovskite Solar Cells

Hybrid organic-inorganic perovskites have been investigated for their potential to serve in next-generation perovskite solar cells (PSCs). While PSC technology is approaching commercialization, thermal and moisture stabilities remain a concern. Here, we describe the assembly of PSCs using an imidazoanthraquinone derivative (AQ) as a small organic additive to enhance the device performance and stability. Unlike polymer additives, AQ is easy to synthesize and is more economical. AQ was synthesized because it has both carbonyl and imidazole functional groups. The presence of C=O and N-H groups results in coordination interaction with Pb2+ and I- of the perovskite. Addition of the AQ molecule to methylammonium lead iodide leads to the formation of a superior crystalline perovskite film with fewer defects and enhanced stability under humid conditions. The use of optimized perovskite films enhanced device power conversion efficiency (PCE = 17.21%) compared to pristine perovskite (PCE = 13.88%). Unencapsulated optimized devices retained 90% of the initial power conversion efficiency for 30 days at a relative humidity of nearly 35%. The optimized films also exhibited superior thermal stability to that of pristine perovskite films.

Design of a CH3NH3PbI3/CsPbI3-based bilayer solar cell using device simulation

With lead-based light harvesters, perovskite solar cells (PSCs) have an efficiency of approximately 25.5%, making them a viable photovoltaic technology. The selection of the absorber materials for PSC in this work are (i) Cesium lead iodide (CsPbI₃) with a 1.73eV bandgap as the first absorber layer, this halide imparts higher stability to perovskite solar cells (ii) CH₃NH₃PbI₃ (MAPbI₃) with a bandgap of 1.55eV is selected as the second absorber layer as this material provides better efficiency to the perovskite solar cells. SCAPS-1D simulation software is used to perform an efficiency analysis of perovskite-perovskite CsPbI₃/MAPbI₃ bilayer solar cell. For efficiency optimization of the perovskite-perovskite bilayer solar cell, we have tried to calibrate seven parameters of the cell. These parameters are (i & ii) selection of the electron and hole transport material (iii, iv & v) variation in the: defect density of bulk material, doping concentration and the thickness of absorber layers, (vi) variation in work function of front electrode (vii) varying interface defect density. After optimization, the efficiency (η) of bilayer PSC is estimated to be 33.54%. The other PV parameters observed in optimal efficiency condition are open-circuit voltage (V_OC) = 1.34V, short-circuit current density (J_SC) = 27.45 mA/cm² and fill factor (FF) = 90.49%. The CsPbI₃/MAPbI₃ bilayer perovskite solar cell efficiency is roughly double the efficiency of single junction CsPbI₃ or MAPbI₃ PSC. Our analysis observed that the variation in the doping and defect density of narrow bandgap material profoundly impacts the efficiency of perovskite-perovskite bilayer solar cells compared to the wide bandgap material.

An Efficient Trap Passivator for Perovskite Solar Cells: Poly(propylene glycol) bis(2-aminopropyl ether)

Perovskite solar cells (PSCs) are regarded as promising candidates for future renewable energy production. High-density defects in the perovskite films, however, lead to unsatisfactory device performances. Here, poly(propylene glycol) bis(2-aminopropyl ether) (PEA) additive is utilized to passivate the trap states in perovskite. The PEA molecules chemically interact with lead ions in perovskite, considerably passivate surface and bulk defects, which is in favor of charge transfer and extraction. Furthermore, the PEA additive can efficiently block moisture and oxygen to prolong the device lifetime. As a result, PEA-treated MAPbI3 (MA: CH3NH3) solar cells show increased power conversion efficiency (PCE) (from 17.18 to 18.87%) and good long-term stability. When PEA is introduced to (FAPbI3)1-x(MAPbBr3)x (FA: HC(NH2)2) solar cells, the PCE is enhanced from 19.66 to 21.60%. For both perovskites, their severe device hysteresis is efficiently relieved by PEA.

Progress in Materials Development for the Rapid Efficiency Advancement of Perovskite Solar Cells

The efficiency of perovskite solar cells (PSCs) has undergone rapid advancement due to great progress in materials development over the past decade and is under extensive study. Despite the significant challenges (e.g., recombination and hysteresis), both the single-junction and tandem cells have gradually approached the theoretical efficiency limit. Herein, an overview is given of how passivation and crystallization reduce recombination and thus improve the device performance; how the materials of dominant layers (hole transporting layer (HTL), electron transporting layer (ETL), and absorber layer) affect the quality and optoelectronic properties of single-junction PSCs; and how the materials development contributes to rapid efficiency enhancement of perovskite/Si tandem devices with monolithic and mechanically stacked configurations. The interface optimization, novel materials development, mixture strategy, and bandgap tuning are reviewed and analyzed. This is a review of the major factors determining efficiency, and how further improvements can be made on the performance of PSCs.

Highly efficient inverted perovskite solar cells incorporating P3CT-Rb as a hole transport layer to achieve a large open circuit voltage of 1.144 V

Poly[3-(4-carboxybutyl)thiophene-2,5-diyl] (P3CT) has been noticed as a promising hole transport layer (HTL) for high-performance inverted planar perovskite solar cells (PSCs) due to its excellent stability and relatively high hole mobility. As we all know, the morphology of perovskite films is largely influenced by the substrate materials. Considering the affinity of alkali metal ions Rb⁺ and Cs⁺ with perovskite materials, inverted perovskite solar cells using alkali metal ion (Rb⁺, Cs⁺) doped P3CT (denoted as P3CT-Rb and P3CT-Cs) as the HTLs were investigated in this work. It turned out that the work function (WF) of P3CT-Rb matches well with the valence band of perovskites. The perovskite (MAPbI_3-xCl_x) film deposited on top of the P3CT-Rb film exhibited a dense and uniform morphology with superior crystallinity and few pinholes. Consequently, a high efficiency of 20.52% was achieved for P3CT-Rb HTL-based devices, with an impressive open-circuit voltage (V_oc) of 1.144 V and a high fill factor (FF) of 82.78%.

The charge carrier dynamics, efficiency and stability of two-dimensional material-based perovskite solar cells

Recent advances in the use of two-dimensional (2D) materials for perovskites solar cells (PSCs) are summarized. The effects of their unique optical and electrical properties on the charge carrier dynamics of PSCs are detailed.

Direct synthesis of high-quality perovskite nanocrystals on a flexible substrate and deterministic transfer

Solid-state perovskite nanocrystals are promising coherent light sources, as there is optical feedback within the crystal structure. In order to utilize the high performance of perovskites for on-chip applications, or observe new physical phenomena, these crystals must be integrated with pre-fabricated electronic or photonic structures. However, the material's fragility has made the deterministic transfer a great challenge thus far. Here, we report the first deterministic transfer of perovskite nanocrystals with sub-micron accuracy. Cesium lead halide (CsPbI₃) nanocrystals were directly synthesized on flexible polydimethylsiloxane (PDMS) stamps via chemical vapor deposition (CVD) and subsequently transferred onto arbitrary substrates/structures. We demonstrated the transfer of a CsPbI₃ crystalline nanoplate (NP) onto an 8 µm fiber core and achieved single-mode whispering gallery mode lasing. Our method can be extended to a variety of other arbitrary substrates (e.g., electrodes, photonic structures, micromechanical systems), laying the foundations for previously unattainable opportunities in perovskites-based devices.

A Cryogenic Process for Antisolvent-Free High-Performance Perovskite Solar Cells

A cryogenic process is introduced to control the crystallization of perovskite layers, eliminating the need for the use of environmentally harmful antisolvents. This process enables decoupling of the nucleation and the crystallization phases by inhibiting chemical reactions in as-cast precursor films rapidly cooled down by immersion in liquid nitrogen. The cooling is followed by blow-drying with nitrogen gas, which induces uniform precipitation of precursors due to the supersaturation of precursors in the residual solvents at very low temperature, while at the same time enhancing the evaporation of the residual solvents and preventing the ordered precursors/perovskite from redissolving into the residual solvents. Using the proposed techniques, the crystallization process can be initiated after the formation of a uniform precursor seed layer. The process is generally applicable to improve the performance of solar cells using perovskite films with different compositions, as demonstrated on three different types of mixed halide perovskites. A champion power conversion efficiency (PCE) of 21.4% with open-circuit voltage (V_OC ) = 1.14 V, short-circuit current density ( J_SC ) = 23.5 mA cm^-2 , and fill factor (FF) = 0.80 is achieved using the proposed cryogenic process.

Investigating Recombination and Charge Carrier Dynamics in a One-Dimensional Nanopillared Perovskite Absorber

Organometal halide perovskite materials have become an exciting research topic as manifested by intense development of thin film solar cells. Although high-performance solar-cell-based planar and mesoscopic configurations have been reported, one-dimensional (1-D) nanostructured perovskite solar cells are rarely investigated despite their expected promising optoelectrical properties, such as enhanced charge transport/extraction. Herein, we have analyzed the 1-D nanostructure effects of organometal halide perovskite (CH₃NH₃PbI_{3- x}Cl _x) on recombination and charge carrier dynamics by utilizing a nanoporous anodized alumina oxide scaffold to fabricate a vertically aligned 1-D nanopillared array with controllable diameters. It was observed that the 1-D perovskite exhibits faster charge transport/extraction characteristics, lower defect density, and lower bulk resistance than the planar counterpart. As the aspect ratio increases in the 1-D structures, in addition, the charge transport/extraction rate is enhanced and the resistance further decreases. However, when the aspect ratio reaches 6.67 (diameter ∼30 nm), the recombination rate is aggravated due to high interface-to-volume ratio-induced defect generation. To obtain the full benefits of 1-D perovskite nanostructuring, our study provides a design rule to choose the appropriate aspect ratio of 1-D perovskite structures for improved photovoltaic and other optoelectrical applications.

The Impact of Atmosphere on the Local Luminescence Properties of Metal Halide Perovskite Grains

Metal halide perovskites are exceptional candidates for inexpensive yet high-performing optoelectronic devices. Nevertheless, polycrystalline perovskite films are still limited by nonradiative losses due to charge carrier trap states that can be affected by illumination. Here, in situ microphotoluminescence measurements are used to elucidate the impact of light-soaking individual methylammonium lead iodide grains in high-quality polycrystalline films while immersing them with different atmospheric environments. It is shown that emission from each grain depends sensitively on both the environment and the nature of the specific grain, i.e., whether it shows good (bright grain) or poor (dark grain) luminescence properties. It is found that the dark grains show substantial rises in emission, while the bright grain emission is steady when illuminated in the presence of oxygen and/or water molecules. The results are explained using density functional theory calculations, which reveal strong adsorption energies of the molecules to the perovskite surfaces. It is also found that oxygen molecules bind particularly strongly to surface iodide vacancies which, in the presence of photoexcited electrons, lead to efficient passivation of the carrier trap states that arise from these vacancies. The work reveals a unique insight into the nature of nonradiative decay and the impact of atmospheric passivation on the microscale properties of perovskite films.

Stable and Efficient Organo-Metal Halide Hybrid Perovskite Solar Cells via π-Conjugated Lewis Base Polymer Induced Trap Passivation and Charge Extraction

High-quality pinhole-free perovskite film with optimal crystalline morphology is critical for achieving high-efficiency and high-stability perovskite solar cells (PSCs). In this study, a p-type π-conjugated polymer poly[(2,6-(4,8-bis(5-(2-ethylhexyl) thiophen-2-yl)-benzo[1,2-b:4,5-b'] dithiophene))-alt-(5,5-(1',3'-di-2-thienyl-5',7'-bis(2-ethylhexyl) benzo[1',2'-c:4',5'-c'] dithiophene-4,8-dione))] (PBDB-T) is introduced into chlorobenzene to form a facile and effective template-agent during the anti-solvent process of perovskite film formation. The π-conjugated polymer PBDB-T is found to trigger a heterogeneous nucleation over the perovskite precursor film and passivate the trap states of the mixed perovskite film through the formation of Lewis adducts between lead and oxygen atom in PBDB-T. The p-type semiconducting and hydrophobic PBDB-T polymer fills in the perovskite grain boundaries to improve charge transfer for better conductivity and prevent moisture invasion into the perovskite active layers. Consequently, the PSCs with PBDB-T modified anti-solvent processing leads to a high-efficiency close to 20%, and the devices show excellent stability, retaining about 90% of the initial power conversion efficiency after 150 d storage in dry air.

Characterization of Low-Frequency Excess Noise in CH3NH3PbI3-Based Solar Cells Grown by Solution and Hybrid Chemical Vapor Deposition Techniques

In this study, detailed investigations of low-frequency noise (LFN) characteristics of hybrid chemical vapor deposition (HCVD)- and solution-grown CH₃NH₃PbI₃ (MAPI) solar cells are reported. It has been shown that LFN is a ubiquitous phenomenon observed in all semiconductor devices. It is the smallest signal that can be measured from the device; hence, systematic characterization of the LFN properties can be utilized as a highly sensitive nondestructive tool for the characterization of material defects in the device. It has been demonstrated that the noise power spectral densities of the devices are critically dependent on the parameters of the fabrication process, including the growth ambient of the perovskite layer and the incorporation of the mesoscopic structures in the devices. Our experimental results indicated that the LFN arises from a thermally activated trapping and detrapping process, resulting in the corresponding fluctuations in the conductance of the device. The results show that the presence of oxygen in the growth ambient of the HCVD process and the inclusion of an mp-TiO₂ layer in the device structure are two important factors contributing to the substantial reduction in the density of the localized states in the MAPI devices. Furthermore, the lifetimes of the MAPI perovskite-based solar cells are strongly dependent on the material defect concentration. The degradation process is substantially more rapid for the devices with higher initial defect density compared to the devices prepared under optimized conditions and structure that exhibit substantially lower initial trap density.

Two-dimensional halide perovskite nanomaterials and heterostructures

Novel two-dimensional halide perovskite nanomaterials and heterostructures enable next generation high performance electronics and photonics.

Cadmium-doped flexible perovskite solar cells with a low-cost and low-temperature-processed CdS electron transport layer

Hybrid perovskite solar cells (PSCs) are promising candidates in exploring high performance flexible photovoltaics, where a low-temperature-processed metal oxide electron transfer layer (ETL) is highly preferable.